_config.yml: Don't explicitly specify markdown engine.

[dmitshur]

Fixes #49.

This is not tested. Done based on what the email said in https://github.com/gopherjs/gopherjs.github.io/pull/48#issuecomment-187121291, to remove specifying markdown option in the _config.yml.

I don't think we're using any subtle features, so the default markdown engine, whatever it may be, should be okay.

[neelance]

Yeah. I'm okay with merging and then checking the output.

[dmitshur]

Let's try. Will revert if I spot any issues.

[dmitshur]

I see some minor differences, but no major issues.

Before:

After:

The largest minor issue is that code blocks have a strange 0-width left padding.

[dmitshur]

On the plus side, no more warning email!

[dmitshur]

FWIW, the HTML before:

And after:

[neelance]

Seems like the CSS file needs an update.